Tunable microminiaturized microwave filters

ABSTRACT

Integrated microwave filters are designed by depositing a convenient strip pattern on a substrate made at least partly of a ferrite material with a permeability varying with the applied external magnetic field. The resonance frequency of the filter is varied across a broad frequency band by varying said external field.

'- Unlted States Patent 1151 3,681,716 Chiron et al. {451 Aug. 1, 1972 [54] TUNABLE MICROMINIATURIZED [56] References Cited MICROWAVE FILTERS UNITED STATES PATENTS [72] Inventors: Bernard Chlron; Loms Dufiau, both f Paris, Frame 3,448,409 6/1969 Moose et al ..333/73 X 3,456,213 7/1969 Hershenov ..333/l.l [731 Assgnee= 5mm Telegraph'qm EL 3,355 680 11/1967 Saltzman et al ass/1.1

Telephoniques, Pans, France Filed: June 16, 1970 Primary Examiner-Paul L. Gensler [21] Appl NOJ 46,597 A1torney-Kemon, Palmer & Estabrook F A h h Pr D [57] ABSTRACT t [30] orelgn pp canon Ion y a 3 Integrated microwave filters are designed by deposit- June 18, France a convenient pattern on a substrate made at least partly of a ferrite material with a permeability [52] US. Cl. ..333/73 S, 333/84 M varying i h the applied external magnetic fi 1 The [51] Int. Cl. ..H0lp 1/20 resonance frequency of the filter i varied across a [58] Flew of Search "333/11, 33 75 broad frequency band by varying said external field.

3 Claims, 11 Drawing Figures PATENTED sum 1 OF 3 AEL Figure 1 PATENTEDAUG 1 I972 SHEET 3 BF 3 FGHz BACKGROUND OF THE INVENTION The general trend in microwave circuit manufacturing is towards reduction of weight and volume. It has led to the development of what is usually called strip" and microstrip technologies. These types of circuits are based on the property of thick metal film conductors coating an insulating substrate to guide microwave energy with very small loss. In the strip technology, tape shaped conductors are deposited on one face of a dielectric support, the opposite face of which is entirely backed with a metal coating. Three plate circuits have also been developed in which tape metal conductors are embedded in an insulator which carries two metal plates on its opposite faces. The present invention concerns strip circuits. It is mainly directed to an improvement of high power tunable filters. By high power is meant mean powers of a few watts and peak powers of several kilowatts. By tunable is meant a filter the cutoff frequency of which can be shifted along a frequency band covering about one octave.

PRIOR ART it has already been proposed to use the anisotropy of magnetic materials in strip circuit manufacture consisting in the replacement of the dielectric substrate by a magnetic material with very poor conductivity. Nonreciprocal properties thus developed are based on the gyromagnetic resonance and the differential phase shift which have been often described.

The variation of ferrite material characteristics with variable magnetic field have already been used for the design of micro strip circuits. For instance U.S. Pat. application Ser. No. 780,120 filed Nov. 29, 1968, (now U.S. Pat. No. 3,560,892) for Improved microwave ferrite devices is based on the variation of the permeability of ferrite materials in order to widen the frequency band of a non-reciprocal device. The present invention is based on the use of the variation of the permeability of magnetic materials in order to tune the resonance frequency of microstrip circuit elements which constitute a filter so as to shift the frequency band of this filter.

The study of fixed frequency microstrip filters has been carried on at several locations. One of the publications on this subject has been issued by McGraw Hill in 1967 under the title Microwave filters impedance matching networks and coupling structures by G.L. MA'I'II-IAEI, Leo Young & E.M.T. JONES. The particular problem of the design of tuned filters is mentioned on pages 472 to 475.

SUMMARY OF THE INVENTION The tunable filters according to the invention are made of a conductive pattern of strips operating as a filter and deposited on a substrate made at least partly of an adjustable permeability ferrite material, two plugs connected to the pattern, a continuous conductive backing for said substrate and external means for establishing an adjustable D.C. magnetic field within said ferrite part of said substrate. The filter is a one or multicell pattern according to current practice. One of such well known patterns as described in the above mentioned book consists in three parallel strips for each cell of the filter. According to the invention the substrate for such strip pattern is wholly or partly made of a ferrite material the permeability of which is sensitive to an external magnetic field. When the substrate is partly made of ferrite material, the substrate underlaying at least part of the central strip of at least one cell is made of ferrite. When the conductive pattern consists of a disk and two diametrally opposed strips, the ferrite underlies the disk shaped conductor.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be fully understood by reference to the following description and the accompanying drawings in which FIGS. 1, 2, 3, 8 and 9 show plan views of filters according to the invention.

FIGS. 4 and 10 show cut-views of filters according to the invention.

FIG. 5 shows the variation of the permeability of a ferrite material with respect to an external D.C. magnetic field.

FIGS. 6 and 7 show the transmission characteristics of a filter according to FIG. 1.

FIG. 11 shows the transmission characteristics of the filter of FIGS. 9 and 10.

DETAILED DESCRIPTION OF THE DRAWINGS In order to describe the invention, reference will be made to a one cell filter. As already mentioned, this should not be considered as a limitation of the scope of the invention which applies to any type of passive filter design, the impedance of which varies with frequency. It is also well known that some filter characteristics require the use of several identical cells to be established instead of only one cell as will be described.

This is fully explained in the above mentioned book by Matthaei et al.

The design of a particular filter required to match a given transmission characteristic is made by application of a mathematical method which is well known to those skilled in the art and fully described in the abovementioned book.

Calculations will set the length, the width and the relative location the conductive pattern elements coating one face of the substrate such as strips. They are made of thick conductive films (a few tens of microns) which are deposited on the substrate by any high precision technology such as silk screen or enrichment through electrolytic deposition of'a thin fihn design obtained by photoetching and so on. FIG. 1 shows a first embodiment of the filter consisting of three strips 1, 2, 3 coating a homogeneous magnetic material substrate 4. Strips 1 and 3 are the input and output strips of the filter and are connected to coaxial plugs not shown. Their width is selected so as to provide impedance matching with the plugs. The underface of the substrate is entirely coated with a metal sheet which does not appear on the top view. The dimensions of strip 2 are chosen according to the above cited mathematical process in order to obtain the filtering characteristic required. External means, not shown on the figure, are provided to establish a DC. magnetic field within the substrate either perpendicular to the plane of the strips or along this plane and parallel to the width of the strips.

FIG. 2 shows an embodiment in which the strips 1, 2, 3 are deposited and a composite substrate made of three parts 5, 6 and 7. The central part 6 is made of magnetic material as was the substrate 4 in the preceding embodiment. Parts 5 and 7 are made of a non-magnetic dielectric material with low microwave losses such for instance as alumina. As shown, the medium strip 2 lies entirely on part 6 of the substrate. Input and output strip conductors l and 3 are deposited on the dielectric parts of the substrate.

On the third embodiment shown on FIG. 3, the substrate is made of a central part 6' of magnetic material and external parts 5 and 7' are made of dielectric material. As shown, the central part 6' of the substrate is shorter than the central strip 2 of the filter. An external means not shown on the drawing establishes a D.C. magnetic field within the substrate part 6'. The central strip 2 of the filter lies partly on the magnetic material 6' and partly on the dielectric material 5'. This feature provides an additional design parameter which consists of the ratio between L and L that is the respective length of the central strip 2 on the dielectric 5 and the magnetic 6 parts of the substrate. The calculations of this circuit based on the theory mentioned in the above cited book allows one to obtain the resonance frequency of a design according to FIG. 1 or 2, where the central strip 2 rests on an homogeneous substrate. In the case of the embodiment of FIG. 3 the electrical continuity of the circuit is obtained provided that in the plane where 5' and 6 meet the admittance of the circuit is zero. In other words, the admittance of that part of the filter which is coated on the magnetic material should be equal but of opposite sign to the admittance of that part of the filter coating the dielectric part of the substrate. This can be written as where A is the wavelength in the air 6, permittivity of dielectric c and ,u permittivity and permeability of the magnetic material 6'.

As shown, the wavelength is related to the permeability of the substrate and will therefore vary when the permeability changes.

FIG. 4 shows a sectional view of the above embodiments of the invention along a plane perpendicular to the substrate faces and to the length of the strips in which 2 refers to the section of the central strips, 6 shows the substrate and 10, the continuous metal coating of the underface of the substrate. The housing which usually carries the connecting plugs has not been shown. The external means for establishing the D.C. magnetic field within the substrate is shown as consisting of the two permanent magnets 11 and 11' associated with two electromagnets made of a ferrite core 13 and 13' surrounded by respectively coils l4 and 14'. The two coils are series connected. The D.C. field due to the permanent magnets is selected so as to maintain the resonance frequency at the center of the band. Ac cording to the direction of the current flow in the coils, the electro magnet field will be added to or substracted from the permanent magnetic field. A ferrite shunt l5 closes the magnetic circuit in order to prevent any loss of magnetic energy and stray magnetic fields.

FIG. 5 shows the variation of the effective permeability of the commercially available ferrite sold by TRANS-TECH under the type reference G. 1004 with respect to the magnetic field, the measurement being carried at 7 GHz. Between points A and B of the curve, the variation law is monotonic and decreasing with increasing magnetic field value. As is obvious, part CD of the curve could also be used, but for technological reasons, it is obviously preferred to use the lower field part of the curve.

FIG. 6 shows the measured resonance curve of a filter according to FIG. I designed with the following data:

strips I and 3 length .65 mm width .60 mm strip 2 length 7.62 mm width .38 mm The curve in FIG. 6 is obtained with a substrate made of G. 1004 ferrite from TRANS-TECH with an external applied field of 2,000 Oersteds (refer to FIG. 5). As shown, the bandwidth at 3 dB is about 200 MHz and attenuation at the resonance frequency about 1.5 dB. The characteristics of this filter do not change when applied power reaches a few watts.

FIG. 7 shows the variation of the resonance frequency of the above filter with the external magnetic field. As has been measured in the frequency band between 6.5 and 7.5 GHz, the bandwidth and the attenuation remain substantially the unchanged.

FIGS. 8 and 9 show two filter designs using microstrip technology. The central conductors 2 and 2 respectively are capacitatively coupled with the external conductors which are designed as impedance matching sections shown respectively at 1 3 and 1 3 Such patterns are well known from the teaching of Matthaei. According to the invention, the substrate is at least partially made of ferromagnetic material in the region which underlies central conductor 2.

FIG. 10 shows a cut view of a filter design according to the pattern shown in FIG. 8. The characteristics of this filter are represented in FIG. 11. Insertion loss a (interrupted line) and the central operating frequency (continuous line) are given for different values of D.C. magnetic field.

As shown in FIG. 10, the substrate 4 supports on its upper face the conductive pattern and lies in a casing 22 made of an aluminum alloy which defines with the upper face of the substrate an airfilled cavity. Casing 22 acts as the conductive plane shown at 10 in FIG. 4. Two mild steel wafers 20 and 21 are set in openings of casing 22 so as to concentrate the magnetic field in the vicinity of the central conductor 2 of the conductive pattern. The magnetic field is established by the two permanent magnets 11 and 11' associated with the electromagnets l4 and 14'. Shield 23 made of mild steel concentrates the magnetic field around the circuit. Casing 22 carries two output connectors which have been omitted for the sake of clarity. Permanent magnets 11 and 11' establish within conductor 2 a magnetic field chosen so that the central operating frequency of the filter corresponds to the lower extremity of the band in which the filter is tunable. Additional field due to electromagnets l4 and 14 is added to the permanent magnet field so as to bring the central frequency of the filter to the desired value. The actual dimensions of the filter design are as follows L= 8.5 mm

1 4.2 mm Thickness of substrate 4 is 1.0 mm. It is made of a ferrite supplied by Lignes Telegraphiques et Telephoniques under the trade type number 6307. The magnetic field due to magnets 11 and 11 is 1,800 Oersteds. The maximum value of the magnetic field is 4,000 Oersteds. The tuning range of the filter is 6.2 to 9 Gl-lz as shown in FIG. 11. In this frequency range, insertion loss remains lower than 5 dB.

What we claim:

1. A tunable microwave filter comprising in combination:

a. a composite substrate including a first rectangular dielectric ferromagnetic part and at least one second rectangular dielectric non-ferromagnetic part, said first and second parts being arranged side by side in contact with each other;

b. a conductive pattern formed of at least three thick film conductors located side by side on one face of said substrate, capacitively coupled to each other, the central one of said conductors extending at least partially over both parts of said substrate;

c. a continuous conductive sheet on the face of said substrate opposite said one face;

d. connecting plugs connected to the other two of said film conductors;

e. external means for establishing an adjustable D.C. magnetic field within said ferromagnetic substrate part; and

f. magnetic shielding means for concentrating said magnetic field within said substrate.

2. A tunable microwave filter according to claim 1 in which said means for establishing an adjustable D.C. magnetic field consists in two permanent magnets located on both sides of said substrate and two electromagnets located on both sides of said substrate serially connected to an adjustable current feed.

3. A tunable microwave filter comprising in combination:

a. a composite substrate including a first part of insulating dielectric non-magnetic material and a tionship being such that Ian (27TI41 tam (271412 VMGQ) A0 i 2 A0 where:

M, is the wavelength in air of the energy propagating in the filter,

e, is the permittivity of the dielectric of said substrate first part,

6 and [L are respectively the permittivity and the ermeab'lit of sai s bst e se t, 21nd l ai'e i especti ve l y th lengt' l i sgi d second strip extending on said substrate first and second P (1. external means for establishing an adjustable D.C. magnetic field within said substrate second part; e. shielding means for concentrating said magnetic field within said substrate; and f. two connecting plugs respectively connected to said first and third thick film strips. 

1. A tunable microwave filter comprising in combination: a. a composite substrate including a first rectangular dielectric ferromagnetic part and at least one second rectangular dielectric non-ferromagnetic part, said first and second parts being arranged side by side in contact with each other; b. a conductive pattern formed of at least three thick film conductors located side by side on one face of said substrate, capacitively coupled to each other, the central one of said conductors extending at least partially over both parts of said substrate; c. a continuous conductive sheet on the face of said substrate opposite said one face; d. connecting plugs connected to the other two of said film conductors; e. external means for establishing an adjustable D.C. magnetic field within said ferromagnetic substrate part; and f. magnetic shielding means for concentrating said magnetic field within said substrate.
 2. A tunable microwave filter according to claim 1 in which said means for establishing an adjustable D.C. magnetic field consists in two permanent magnets located on both sides of said substrate and two electromagnets located on both sides of said substrate serially connected To an adjustable current feed.
 3. A tunable microwave filter comprising in combination: a. a composite substrate including a first part of insulating dielectric non-magnetic material and a second part of ferromagnetic material, said parts being arranged in side by side contiguous relation; b. a continuous conductive plate on a first face of said substrate; c. a conductive thick film pattern on a face of said substrate opposite said first face, said pattern having at least three capacitively coupled strips, the first and third of which serve as matching sections and the second as a resonator, said second strip being disposed over both substrate parts, the relationship being such that where: lambda o is the wavelength in air of the energy propagating in the filter, epsilon 1 is the permittivity of the dielectric of said substrate first part, epsilon 2 and Mu are respectively the permittivity and the permeability of said substrate second part, L1 and L2 are respectively the length of said second strip extending on said substrate first and second parts; d. external means for establishing an adjustable D.C. magnetic field within said substrate second part; e. shielding means for concentrating said magnetic field within said substrate; and f. two connecting plugs respectively connected to said first and third thick film strips. 